Intercarrier noise suppressor system



19, 1941- P. DLLowELL $252,811

INTERCARRIER NbISE SUPPRESSOR SYSTEM Filed July 29, 1940 s Sheets-Sheet 1 Jana HRH/fer I anti/la for AUd/d I. I INVENTOR. 1 -|l'- PERCIVAL D. LOWELL BY Q ATTORNEY A g- 19, 1 4 P. D. LOWELL ,811

INTERCARRIER NOISE SUPPRESSOR SYSTEM Filed July 29,'l940 3 Sheets-Sheet 2 v PFi/far Jaaa N PERC IVAL D. LOWELL Aug. 19, 1941.

P. D. LOWELL INTERCARRIER NOISE SUPPRESSOR SYSTEM Filed Jul 29, 1940 3 Sheets-Sheet s A plifier 55 Grid: 43

F Z 37 INVENTOR.

' g PERCIVAL D.LOWEI L.

ATTORNEY Patented Aug. 19, 1941 UNITED STATES PATENT OFFICE INTERCARRIER NOISE SUPPRESSOR SYSTEM Percival D. Lowell, Chevy Chase, Md. Application July 29, 1940, Serial No. 348,308

6 Claims.

an automatic noise suppression system with automatic signal threshold control for maximum clarity in signal receiving systems.

Still another object of my invention is to provide an automatic noise suppression system and combined automatic volume control system for complete regulation of the audible output of a signal receiving system, the signal threshold being automatically controlled through the noise suppression system.

A further object of my invention is to provide an automatic noise suppression system coupled with the normal automatic volume control system of a signal receiving circuit, with automatic signal threshold control through the noise suppression system.

' A still further object of my invention is to provide an automatic noise suppression system for a pair of signal receiving circuits with mutually interrelated control circuits for noise level and signal threshold control and automatic cutoff of one of the receiving circuits upon the reception of a signal in the other receiving circuits.

Other and further objects of my invention reside in the arrangement and operation of the various circuits hereinafter described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a superheterodyne radio receiving circuit embodying a combined automatic volume control and. intercarrier noise suppressor system in accordance with my invention; Fig. 2 is a schematic diagram of a radio receiving system having an automatic volume control circuit and embodying an inter-carrier noise suppressor system in accordance with my invention; Fig. 3 is a schematic diagram indicating a modification of the system of Fig. 2, wherein the noise suppressor system of my invention is employed in connection with a different type of volume control circuit; Fig. 4 is a schematic diagram of a two receiver system embodying inter-carrier noise suppressor systems mutually interconnected with the two receiving circuits for automatic cut-off and signal threshold control, where each receiver is proe vided with automatic volume control means of one type; and Fig. 5 is a schematic diagram indicating a modification of the system of Fig. 4, wherein the noise suppressor systems of my invention are employed in connection with volume control circuits of a different type.

In its primary aspects, the noise suppressor systemof my invention involves the extraction of audible noise components from an output circuit for application as a control factor to vary the gain of related circuits, inaccordance with the particular embodiment of the invention and the purposes to which it is adapted. For single channel radio receiving systems, such as are shown in Figs. 1-3, the noise suppressor system is applied to control the level of intercarrier noise, and at the same time provide an automatic signal threshold control, in the receiving system from which the controlling noise components are derived. But the system ofmy invention is not limited thereto, as shown by the further embodiments illustrated in Fig. 4 and 5 wherein noise components derived from one receiving system are applied to a coacting, related receiving system, and vice versa, for selective operation of one or the other receiving system and mutual control of sensitivity in the two receiving systems. My invention, therefore, is directed'to various features relating to noise suppression in electrical sound reproducing systems, as will be understood from the specific embodiments hereinafter described in detail.

Fig. 1 discloses the application of my invention to a system for obtaining automatic volume control and at the same time providing for automatic control of the radio frequency gain as the noise level varies in a superheterodyne radio receiver. The noise level automatically biases the radio frequency grids negative and keeps the gain down to the point where the noise in the sound reproducer is substantially reduced in proportion to the noise level itself, while a continuous carrier signal which is equal to or stronger than the noise level may be reproduced but also automatically controlled in amplitude so as not to exceed a predetermined value.

Referring to Fig. l, in detail, I have illustrated my invention in somewhat simplified form, applied to a superheterodyne receiver comprising radio frequency amplifier I, first detector 2, frequency converting oscillator 3, intermediate frequency amplifier 4, second detector 5, and audio frequency amplifier 6. The oscillator 3, besides generating the usual high frequency oscillations,

is also audio modulated by an approximately 5,500 cycle wave from a source at I. Interposed between the second detector 5 and the audio amplifier 6 is a low-pass filter 8 designed to cut off sharply all frequencies higher than 5000 cycles but pass all frequencies below 5000 cycles, including voice frequencies, to sound reproducer 9. Connected in parallel with the second detector output is another filter l0, which cuts off sharply all frequencies below 5000 cycles but passes all frequencies above 5000 cycles, including the 5500 cycle modulation from source 1. The output of filter l0 feeds into the auxiliary audio frequency amplifier l l, thence through rectifier l2, shunted by smoothing condenser I3 across its output, and the rectified voltage is introduced into the radio frequency and intermediate frequency grid return circuits.

The operation of the circuit is as follows. Suppose the receiver to be operating and no carrier signal coming in but some noise having a relatively continuous characteristic is received. Upon reaching the output of the second detector 5, the noise will branch into the two filter channels,part of it going into and through high pass filter I!J,amplifier [I and rectifier l2. The noise currents are rectified and smoothed and applied to;the radio frequency and intermediate frequency grids in such a polarity relationship that the rectified noise currents causethe grids to be biased more negatively, thereby reducing the radio frequency and intermediate frequency gain in proportion to the strength of the noise currents.

'Now if a voice modulated carrier signal is tuned in, it combines in the first detector 2 with the approximately 5500 cycle modulated radio frequency oscillations of oscillator 3, and appears in the output of the second detector 5, as voice modulation plus a; steady approximately 5500 cycle modulation. The voice modulation, being composed of frequencies almost entirely below 5000 cycles, passes through the 5000 cycle low pass filter 8, is amplified by audio frequency amplifier 6, and reproducedin the sound reproducer 9, freefrom the approximately 5500 cycle modulation. Filter l0, being 5000 cycle high pass, rejects the voice frequencies but passes the approximately 5500 cycle modulation through to amplifier II and rectifier l2.v The rectified and smoothed 5500 cycle modulation will then bias the radio frequency and intermediate frequency grids negatively, causing the gain of the receiver to vary in proportion to the strength of the signal carrier wave, tending to hold the speech signal impressed upon the sound reproducer at a predetermined level.

The circuit shown in Fig. 2 provides for an automatic "threshold level adjustment which will vary the. sensitivity of the receiver according to the value of the noise level. The circuit is that of a superheterodyne receiver with radio frequency amplifier l5, first detector l6, frequency converting oscillator l'l, intermediate frequency amplifier l8, diode detector and first audio frequency amplifier l9, second audio frequency amplifier 20, and sound reproducer 2!. The circuit includes provisionfor autogmatic volume control by the known method of producing a control voltage across a resistor 22 in the diode detector circuit, upon rectification of the carrier therein, and applying this carrier-supplied voltage as a bias to the grids of the radio and intermediate frequency amplifier tubes. the output of the second detector and audio frequency amplifier I9 is provided through a 5000 A second path from cycle high pass filter 23 to auxiliary audio frequency amplifier 24, thence to rectifier 25 which is shunted by a smoothing condenser 26 across its output. The output of rectifier 25 is introduced into the automatic volume control circuit through potentiometer 21, with its polarity in series relation to that of the control voltage across resistor 22.

The operation of the system of Fig. 2 is as follows. Suppose the receiver is operating and no carrier signal is tuned in but noise having a certain level is present. The noise currents which leave the second detector and audio frequency amplifier iii are applied to filter 23 which is designed to cut off sharply all frequencies below 5000 cycles, and as a great portion of the noise will have a frequency characteristic of 5000 cycles or higher, it will pass through filter 23, be amplified in tube 24, and rectified by rectifier 25. The rectified noise will then be smoothed in condenser 26 and introduced into the automatic volume control circuit through potentiometer 21 and thence into the grid circuits of the radio and intermediate frequency amplifier tubes. The connections from the rectifier 25 are polarized so that the negative from the rectifier output will connect to the grid return circuits. The rectified noise currents will thus cause the amplifier grids to become more negative according to the strength of the noise and the setting of the potentiometer 21 and the amplification will be reduced in proportion to the noise amplitude.

Upon receiption of a modulated carrier wave of sufficient amplitude to cross the threshold imposed by the bias from potentiometer 21, the modulation will carry through from second detector and audio frequency amplifier I 9 to amplifier 20 andsound reproducer 2|. Since substantially all the modulation frequencies are below 5000 cycles, they will not pass through filter 23 and consequently cannot affect the noise control bias delivered from rectifier 25 through potentiometer 21. The volume control functions, ob-

tained in the system of Fig. 1 through the use of an approximately 5500 cycle auxiliary modulation, are separately produced in the diode detector circuit at l9 and by resistor 22 in the system of Fig. 2. In effect therefore, Fig. 2 illustrates a receiver which is free from noise while no carrier is being received, but which will respond to any carrier having an amplitude equal to or greater than the level of the noise.

Fig. 3 shows the modification which is made in the system of Fig. 2 when a separate automatic volume control tube is employed. In Fig. 3, a triode detector is shown at 2B supplyiing noise and modulation currents to audio frequency reproducer 2 I and filter 23' which is identical to filter 23 of Fig. 2. Similarly as in Fig. 2, a noise control bias voltage is delivered from auxiliary amplifier 24', through rectifier 25' and smoothing condenser 26' to potentiometer 21. The automatic volume control tube is shown at 29 with load resistor 30 connected with the anode and having the volume control voltage produced thereacross in the polarity indicated. Resistor 30 and potentiometer 21 are connected in series relation as indicated to deliver automatic volume control (A. V. C.) and inter-carrier noise suppressor (I. N. S.) bias potentials to the grids of preceding amplifiers through lead 3|, similarly as are resistor 22 and potentiometer 2'! in Fig. 2.

The system of my invention has special application at aircraft ground stations where it is desired to have 'two receivers maintaining a Watch on two wave lengths simultaneously. The circuits of Figs. 4 and 5 employ the system of my invention for automatically rendering one re ceiver inoperative during the time the other is receiving a modulated carrier signal, and vice versa. Thenoise present in one receiver automatically controls the sensitivity of the other receiver, and vice versa, when no signals are being received, so that the sensitivity of both. receivers is automatically maintained. at the point where no noise is heard. Either receiver may operate to reproduce a signal, but upon the reception of a signal in one receiver the other receiver is rendered inoperative for the duration of the signal, through the same control circuits.

Referring to Fig. 4, the two receivers are shown to comprise radio frequency amplifiers 32 and 33, respectively, combined diode rectifiers and audio frequency amplifiers 34 and 35, respectively, and sound reproducers 36 and 31, respectively. The diode rectifiers at 34 and 35 serve as detectors for the signal carriers and develop audio signal components across resistors 38 and 30, respectively, which are applied to the grids of the triode portions of tubes 34 and, 35 for audio frequency amplification. The differences of potential developed across resistors 38 and 39 are applied through resistors 40 and 4|, respectively, to the grid electrodes of radio frequency amplifiers 32 and 33, thereby producing an automatic volume control action. Also associated with the audio output circuit are rectifiers 42 and 43. The rectifled audio signals are smoothed by condensers 44 and 45 and applied to the grid electrodes of radio frequency amplifiers 33 and 32, respectively in series with the automatic volume control voltages; that is, the rectifier of one receiver is connected in series with the automatic volume control circuit of the other receiver. Potentiometers 46 and 41 are connected in the outputs of rectifiers 42 and 43, respectively, for adjusting the magnitudes of the control potentials.

The operation of the circuit of Fig. 4 is as follows: Suppose both receivers are energized but no signal is detected in either one. any amount of residual noise in the output of receiver 3240, for example, the audio output of this receiver will be rectified at 42 and effective to bias the radio frequency amplifier of the other receiver negatively, so that the threshold of the other receiver will be controlled according to the noise in the output of the receiver 32-40 and to the setting of the potentiometer 45. Likewise noise in the output of receiver 334l will lower the radio frequency gain in receiver 32--40, and

the net result will be that one receiver Will regulate the amount of noise reproduced by the other. Then if a modulated carrier is present of the frequency to which the receiver 32fi0, for instance, is tuned, a relatively high value of audio voltage will be fed from this receiver into rectifier 42, the output of which will supply a negative cutoff bias to the grid of radio frequency amplifier 33, in the receiver 334l, so that the receiver 334| is incapable of reproducing a signal while the other receiver is in operation. Similarly, the receiver 32-40 will be biased to cut-off by receiver 334l when the signal present is of the frequency to which receiver 334l is tuned.

A modified form of the system of Fig. 4, wherein separate automatic volume control tubes are employed, is shown in Fig. 5. Rectifiers 42' and 43, condensers 44' and 45 and potentiometers 46 and 41' may be the same as those shown in Fig. 4,

If there is and sound reproducers 36' and 31 likewise. ."In lieuof the diode'detecto'rs and amplifiers 34 and 35 of Fig. 4, however, final audio frequency amplifiers 48 and49 are shown in Fig. 5, supplying the output to reproducer 36 and rectifier 42', and to reproducer 31 and rectifier 43, respectively. The automatic volume control means include electron tubes 50 and 5| having load resistors 52 and 53 connected with the anodes thereof, respectively. In order to effect the operations described above in reference to Fig; .4, potentiometer 41' and resistor 52, of respectively different receiver circuits, are connected together with the voltages thereacross in series relation and a connection at 54 to supply control potentials to the grids of previous amplifiers. Similarly, potentiometer 4B and resistor 53 are connected together and to lead 55.

While I have thus described my invention in various embodiments, I desire it understood that if further modifications and adaptations thereof may be made, and that no limitations upon my invention are intended thereby but only such as are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. In combination, a superheterodyne radio receiving system including amplifier means, a local oscillator, first and second detectors and a sound reproducing circuit, means for modulating said localoscillator at a selected frequency above the range of signal frequencies, a low pass filter interposed between the second detector and said sound reproducing circuit and adapted to pass currents of signal frequencies, a high pass filter connected with said second detector and adapted to pass current of said modulation frequency and a substantial proportion of noise components, current of said modulation frequency being detected in said second detector only with the recep tion of a carrier in said radio receiving system, rectifier means connected with said high pass filter for producing direct potentials proportional to the intercarrier noise level in said system and to the magnitude of said carrier, and means for applying said potentials to control the gain in said amplifier means, for providing automatic signal threshold control dependent upon the noise level in the absence of a carrier and automatic volume control dependent upon the carrier level in the presence of a carrier.

2. The combination set forth in claim 1 wherein said modulation frequency is approximately 5500 cycles per second, said low pass filter has an upper limit of 5000 cycles per second whereby currents of voice frequencies may be passed, and said high pass filter has a lower limit of 5000 cycles per second whereby said approximately 5500 cycle modulation and the greater portion of noise components in the output of said second detector may be passed to said rectifier means.

3. In combination, a radio receiving system including amplifier means, a detector energized by said amplifier means and a sound reproducing circuit, means responsive to noise components in said system including a high pass filter connected with said detector and adapted to pass a substantial proportion of noise components, a rectifier connected with said filter and operative to produce a direct potential proportional to the noise level in said system, and gain control means energized by said potential to control the gain in said amplifier means for providing automatic signal threshold control dependent upon the noise level in the absence of a carrier.

4. The combination set forth in claim 3 Wherein said receiving system includes means for producing a second direct potentialproportional to the average signal strength for automatic volume control, and said gain control means is energized also .by said second direct potential forautomatic volume control. I 5. The combinationset forth in claim 3 wherein said detector comprises a diode arrangement having volume control means energized therefrom and effective on said amplifier means, and the said gain control means energized by the potent-ial dependent upon noise level is cooperative with said automatic volume control means.

6-. The combination set forth in claim 3 wherein said receiving system includes automatic volume control means comprising a triode electron tube and a resistor connected therewith having a potential drop proportional to the carrier level and effective on said amplifier means, and the said gain control means energized by the potential dependent upon noise level is cooperative with said automatic volume control means. i PERCIVAL D. LOWELL. 

